With the ever-increasing popularity of mobile and portable electronic devices, demands for small and low cost pointing device have increased dramatically. Optical navigation technologies used inside optical mice offer many advantages over other solutions in accuracy, reliability and flexibility. However existing mouse sensors modules are too big to be applied to portable electronic devices such as cellphones and personal digital assistants (PDAs).
Optical mouse sensors detect relative motion by tracking the changes in the captured images from the reflection off the navigation surface. The navigation images may be patterns of shadows and highlights on the surface cast by an obliquely illuminated light emitting diode (LED), or diffraction patterns or speckles formed by the navigation surface with a coherent illumination source. The heart of an optical mouse is the navigation module that consists of a light source, an image sensor and an optical piece (may or may not contain a optical piece), e.g. an imaging lens. The light source, e.g. a LED or a laser, is used to provide illumination of the navigation surface; while the imaging portion of the optical piece forms navigation images onto the image sensor array. An imaging lens typically is used to image the navigation images onto a 2D sensor array. The size of a typical mouse sensor module is quite large, especially the height. The large module size has not been an issue for the proliferation of the optical navigation technologies in applications such as desktop computer mice. It however prevents the broader applications of the same technology in portable electronic devices such as cellphones and PDAs. For portable electronics, a pointing device with a thickness that is on the order of 2 mm is desired, while a typical mouse sensor module requires more than 10 mm of space between the navigation surface and the sensor.
Existing optical mouse sensors are too big to be integrated into portable electronic devices such as cellphones, primarily due the height of the device assembly. A typical optical mouse sensor requires more than 14 mm spacing between the navigation surface and the image sensor in order to satisfy the imaging conditions required for navigation. According to the current module design, the imaging function is typically performed by a single lens element. The minimum spacing between the surface and the sensor is limited by the sensor pixel size and the array size, optical efficiency (or f/#) and the desired optical magnification.
The height of the traditional mouse sensor module is primarily limited by the height requirement of the image optics. Typically a single plastic molded lens is used with a unit magnification, and a 2D sensor with 20×20 pixels at 50 um pitch is used. In first order, in order to achieve 1:1 imaging, the spacing between the navigation surface and the sensor will be 4× the focal length of the image lens. As the spacing between the navigation surface and the sensor get reduced, the focal length of the imaging lens must be reduced. It is theoretically possible to design a very short focal length lens for this purpose therefore achieving a near 2 mm spacing between the sensor and the navigation surface, although in practice the aperture size of the lens must be considered in order to maintain reasonable light collection efficiency. The ratio of the lens focal length and the diameter of the lens is the f-number of the lens. It is practically difficult to have a single element refractive lens to have an f/# that is smaller than 1 that can cover a large field of view with good image quality. For this reason, a typical optical mouse sensor module requires a more than 10 mm spacing between the sensor and the navigation surface.